Distinguishing power line arcing from RF emissions

ABSTRACT

Specific noise components in an input signal are identified and isolated for processing in a desired manner. The method comprises determining an interval during which the noise component is expected to occur and isolating a portion of the input signal occurring during that interval. Determination of the interval is made by sensing noise impulses in the input signal, temporally correlating the noise impulses, and estimating the time that the noise impulses are expected to recur. The input signal can then be passed during the interval to an output signal to enhance the noise component, or the input signal can be blanked from the output signal to reduce the noise component.

This is a continuation of application Ser. No. 07/949,040, filed Sep.21, 1992, now U.S. Pat. No. 5,499,189.

FIELD OF THE INVENTION

The present invention relates generally to apparatus for filtering aninput signal, and more particularly relates to a method and apparatusfor isolating specific noise components in an input signal so they canbe processed in a desired manner.

BACKGROUND AND SUMMARY OF THE INVENTION

For expository convenience, the present invention will be illustratedwith reference to one particular application thereof, namely that ofsensing sparking noise from electrical power lines. However, it shouldbe recognized that the invention is not so limited.

Electrical power lines carry a signal which is typically a periodic 60Hz AC voltage. In an AC signal, the voltage potential varies as asinusoid between positive and-negative maximum values with periodic zerocrossings.

At the zero crossings, the power line has a voltage potential near thatof ground. At the maximum excursions of the signal, however, there maybe enough of a potential difference between the power line and adjacentobjects to induce a spark. Sparking is most likely to occur with highvoltage power lines, or in settings where the physical integrity of thepower line is compromised (i.e. loose hardware, etc.). However, sparkingcan occur wherever there is a power signal, and is thus a widespreadphenomenon.

Sparks create wideband radio frequency (RF) signals that dan interferewith all manner of radio and television equipment. However, the radiosignals produced by such sparking also provide a means by which thesparking power line can be tracked down and identified. Electric utilitycompanies often have crews of troubleshooters who utilize radioreceivers and directional antennas to locate sparking power lines sothey might be repaired. In this way, interference to radio andtelevision equipment can be traced and eliminated.

A hindrance to such tracing of sparking power lines is that power linesare not the only sources of electrical sparks. Vehicle ignition systems(e.g. "spark" plugs) also produce spark noise that generates similar RFinterference. Due to the alternating voltage of power signals, powerline sparking generally occurs with a repetition rate of 60 Hz (120 Hzin instances where sparking occurs with equal regularity on the positiveand negative excursions). Depending on engine speed, vehicle ignitionsystems can also generate sparks at 60 Hz and multiples thereof. RFinterference from vehicle ignition systems is especially troublesomewhen attempting to trace a sparking power line while traveling in amotor vehicle.

There are some differences between periodic ignition sparks and powerline sparks. Power line sparks typically occur more than once during a60 Hz sinusoidal power signal cycle. Sparks may occur during thenegative half as well as the positive half of the power signal cycle.Also, after a spark occurs, the potential difference is quicklyrecharged to a voltage where sparking may again occur. (The power linemay be modelled as an R-C circuit, with a capacitance formed by aninsulator, and then a large resistance--such as a wooden pole--toground.) Since the recharging is quick, several sparks and rechargesusually occur in succession during the same half cycle of the powersignal. Thus, during each half cycle, a series of several closely spacedsparks typically occur.

In accordance with the present invention, a method and apparatus areprovided for isolating specific noise components in an input signal sothey can be processed in a desired manner. The method comprisesdetermining an interval during which the noise component is expected tooccur, and isolating a portion of the input signal occurring during thatinterval. Determination of the interval is made by sensing noiseimpulses in the input signal, temporally correlating the noise impulses,and estimating the time that the noise impulses are expected to recur.The input signal can then be passed during the interval to an outputsignal to enhance the noise component, or the input signal can beblanked from the output signal to reduce the noise component.

The preferred apparatus comprises a noise impulse detector, a memory, anelectronic switch, and a general purpose processor programmed accordingto the method of the present invention. The noise impulse detectorsenses noise impulses in an input signal. The time that noise impulsesare sensed is recorded by the processor in the memory. The processoralso temporally correlates the sensed noise impulses to determine aninterval during which a particular noise component of the input signalis expected to recur. The processor actuates the electronic switchduring the interval to pass or blank the input signal during theinterval.

The present invention has particular application to the detection ofradio frequency interference or noise impulses caused by power linesparks. As described above, such noise impulses can be expected to occurat intervals correlated to a 60 Hz power signal cycle. Thus, a window oftime during which a power line spark may be expected will occurapproximately one 60 Hz cycle (16.7 milliseconds) after a first powerline spark is detected. Therefore, the present invention can bepracticed by receiving a radio frequency input signal containingwideband noise from power line sparks, detecting a noise impulse,predicting an interval for a next noise impulse (i.e. approximately 16.7milliseconds after the detected noise impulse), and passing the inputsignal during the predicted interval. This particular application of thepresent invention effectively passes power line noise while blankingnon-periodic noise. Of course, non-periodic noise which coincidentlyoccurs during those intervals in which the input signal is passed willnot be blanked.

The foregoing implementation of the invention, however, does notdiscriminate against other sources of 60 Hz noise. As described above,other sources of periodic 60 Hz noise impulses exist, and these sourceswould also be detected by the foregoing technique. Accordingly, themethodology may be further refined by exploiting a unique characteristicof power line sparks, such as their tendency to occur in closely spacedgroups. In such a refined methodology, it is not enough that a secondnoise impulse be detected approximately 16.7 milliseconds after a firstimpulse. Instead, the second noise impulse must be detectedapproximately 16.7 milliseconds after a group of closely spaced firstnoise pulses. Thus implemented, the present invention enhances the radiofrequency interference caused by power line sparks while blankingnon-periodic noise and periodic noise from other sources.

Additional features and advantages of the present invention will be madeapparent from the following detailed description of a preferredembodiment, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an apparatus for blanking non-periodicnoise according to a preferred embodiment of the present invention.

FIG. 2 is a block diagram of a circuit in the apparatus of FIG. 1.

FIG. 3 is a schematic diagram of a portion of the circuit of FIG. 2.

FIG. 4 is a flow chart of basic steps in a process for blankingnon-periodic noise according to a preferred embodiment of the presentinvention.

FIGS. 5a-c are more detailed flow charts of the process illustrated inFIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, and in accordance with a preferred embodimentof the invention, an apparatus 10 for blanking non-periodic noise isprovided. The apparatus is typically used by employees of a utilitycompany to identify a particular location in the field at which a powerline may be sparking. The apparatus is preferably embodied in ahand-held instrument for portability in the field. Accordingly, theapparatus comprises a box-shaped instrument housing 14 which may becarried over the shoulder in a carry case. Alternately, the apparatusmay be embodied as a vehicle-mounted instrument.

The apparatus further comprises a hand-held directional antenna 16 whichis connected to the instrument housing 14 using a coaxial cable 18. Thecoaxial cable attaches to a BNC connector 20 provided on the housing(shown in FIG. 2). The directional antenna can be directed towards anelectrical power line so as to receive a radio frequency signal (theinput signal) containing any interference caused by sparking at thatlocation along the power line.

On a face 24 of the apparatus 10, there is a visual display 26, such asan LCD display, for indicating the frequency to which the apparatus istuned. Also provided is a set of controls including a band selectingswitch 28, a volume control knob 25, a frequency knob 29, a gain knob30, a video level knob 31, a sync knob 32, a digital filter bypassswitch 33, and a meter 70 (FIG. 2).

The apparatus 10 also comprises an audio jack 37, a video connector 34,and a synch connector 35. A set of headphones 38 can be connected to theapparatus by plugging the headphones into the audio jack 37. When theheadphones are connected, the speaker 68 is automatically muted. Theheadphones/speaker provide an audible indication of the radio frequencyinterference caused by power line sparks. The video connector 34 permitsattachment of an oscilloscope or other signal measurement instrument fopanalysis and display of the input signal after processing by theapparatus. A 60 Hz synchronization signal is provided at the synchconnector 35 for such instruments. (Video output is also provided on thetip ring of the audio jack 37.)

Enclosed in the housing 14 of the apparatus 10 is a circuit 44 as shownin FIG. 2. The input signal from the directional antenna 16 is receivedby the circuit at the BNC connector 20. The input signal is amplifiedwith an amplifier 46. A tuner 48, consisting of a variable frequencylocal oscillator and mixer, tunes a selected frequency of the inputsignal as selected by the band selection switch 28 and the frequencyknob 29. A frequency counter 50 determines the frequency of the tunedinput signal and displays the measured value of the frequency on thedisplay 26. Since power line sparks create noise spreading acrossvirtually the entire frequency spectrum, nearly any frequency wouldcontain noise impulses from a power line spark. However, in order tominimize interference from other sources of radio frequency signals (andto avoid possible nulls in the spectral distribution of the noise due tothe sin(x)/x frequency character of a finite noise impulse) it isdesirable to tune to a specific frequency. Tuning of a frequency in thevery high frequency (VHF) range is normally more effective for locatinga group of utility poles that contain the source of power line sparks.Frequencies in the ultra high frequency (UHF) range are tuned toidentify a specific utility pole where the spark source is located.

The tuner 48 provide an intermediate frequency (IF) signal at 45 Mhzhaving a bandwidth of about 6 Mhz. This IF signal is next filtered by aband pass filter 54. The band pass filter operates to limit the rise andfall times of noise impulses in the input signal. The filter output isprovided to an RF detector 56.

Connected after the detector 56 is a digital filter 58. The digitalfilter 58 implements the method for blanking non-periodic noise of thepresent invention. The filter operates to blank non-periodic noise fromthe input signal while passing periodic noise impulses of the typeproduced by power line sparks. Operation of the digital filter 58 isillustrated in FIG. 3 and described in more detail below. The digitalfilter 58 may be disabled by the filter bypass switch 33.

After processing by the digital filter, the input signal should containonly those noise impulses caused by power line sparks. Since the noiseimpulses are of very short duration, they must be re-shaped by audio andvideo pulse shaping circuits 62, 64 to be perceptible using theheadphones 38 or an external oscilloscope. The audio pulse shapingcircuit 62 is preferably implemented as a peak detector with a slowdecay. The processed input signal is amplified by an audio amplifier 66which drives the headphone jack 37 or a built-in speaker 68 with theprocessed input signal. The output of the audio pulse shaping circuit 62is also provided to the built-in meter 70 through a meter amplifier 72.

The video pulse shaping circuit 64 is similar, extending noise impulsesto make them more easily perceptible on an oscilloscope screen. Thevideo pulse shaping circuit also provides an adjustment of the amplitudeof the video signal using video level knob 31. The output of the videopulse shaping circuit is provided to an externally attached oscilloscopethrough the video connector 34.

A synchronizing 60 Hz signal for the oscilloscope is provided by a 60 Hzgenerator 74 at the synch connector 35. The frequency of thesynchronizing signal can be adjusted using the synch knob 32.

Power is provided to the circuit 44 by a rechargeable battery 78 andpower supply circuit 80. The power supply circuit 80 generates variouspower supply voltages required by the circuit. The battery 78 can berecharged from a 12 volt source connected to a charging circuit 82 by acharge connector 84. The condition of the battery 78 is monitored by lowbattery detection circuit 86 and displayed on the LCD display 26.

FIG. 3 illustrates in greater detail the digital filter 58 whichimplements the present invention. A first portion of the circuitperforms the function of detecting a noise impulse in the input signal.The input signal is received at an input 90 to the digital filter 58after being tuned, filtered, and amplified by the circuit 44. A resistor92 and a capacitor 94 are connected in series between the input 90 andground. The resistor and capacitor form a DC voltage at their junction96 equal to the average DC level of the input signal.

The average DC level is used to determine a threshold value in amultiplier circuit 100. The multiplier circuit 100 comprises anoperational amplifier 102 (type TL084) and resistors 104, 105, 106. Theaverage DC level of the input signal is received at a positive input 108of the operational amplifier. The resistors 104, 105, 106 are connectedto the operational amplifier in a well known manner to cause theoperational amplifier to produce at an output 110 a threshold valuesignal which is preferably approximately six times the average DC level.(A small offset voltage, on the order of 20 millivolts, is provided tothe left side of resistor 104.)

The threshold value signal is received at a positive input 112 of acomparator 114 (type LM393), which serves as a threshold level detector.The comparator compares the signal from input 90 to the threshold valueset by the operational amplifier 102. When the input signal exceeds thethreshold value signal, the comparator generates an interrupt signal atan output 122. Since the threshold level is set at about six times theaverage DC level of the input signal, the interrupt signal should onlybe generated when there is a noise impulse in the input signal. Aresistor 124 is connected at the output 122 and operates as a pull upresistor since the comparator has an open collector output.

A further portion of the digital filter 58 operates to temporallycorrelate detected noise impulses with the period of an AC power signaland to estimate intervals during which noise impulses caused by powerline sparks are expected to occur. In the preferred embodiment, thisportion of the digital filter comprises a general purpose processor 126(i.e. Motorola type MC68HC05) programmed to execute the method of thepresent invention, as described in greater detail below in connectionwith the flow charts of FIG. 4. The processor 126 includes an interruptrequest (IRQ) input 130 and a clock input 132. The IRQ input receivesthe interrupt signal generated by the second operational amplifier 114.The microprocessor further includes internally a flip-flop connected tothe IRQ input which stores an interrupt signal until serviced by theprocessor's program. In the preferred embodiment, the clock input isdriven with a 3.6864 MHz clock signal. However, other frequencies can beused for the clock signal if sufficiently fast for the processor totimely complete its programmed process.

In the preferred embodiment, the processor 126 is programmed to serviceinterrupts 120 times at uniform intervals during each cycle of a 60 Hzsignal. Each 60 Hz cycle is approximately 16.7 milliseconds. For eachinterrupt serviced, the contents of the internal flip-flop connected tothe IRQ input 130 are stored at consecutive locations of a data array.In the illustrated embodiment, the data array is maintained in themicroprocessor's built-in RAM memory. (In other embodiments, of course,an externally connected memory addressable by the processor could beused.) In this manner, a history of the times at which noise impulsesoccurred during the previous cycle is maintained.

The processor 126 is further programmed to determine from the data arrayan interval during which a noise impulse caused by power line sparks isexpected to occur. The time that noise impulses from power line sparksare expected to occur is predicted from the times that noise impulsesoccurred during the previous cycle as indicated by the data array. Theprocess for predicting this interval will be made more apparent in thedescription of the flow chart of FIG. 4 below.

The processor also controls a last portion of the digital filter whichoperates to blank periodic noise from the input signal. The processorhas a data output 136 which it drives with a switch control signalaccording to its program. The switch control signal operates anelectronic switch 138 and is received by the switch at a control input140. The electronic switch has a first input 142 at which the inputsignal is received. A second input 144 receives the average DC value ofthe input signal from the juncture 96 of the resistor 92 and thecapacitor 94. During the intervals predicted by the processor foroccurrence of noise impulses from power line sparks, the switch controlsignal operates the switch to connect the input signal to a switchoutput 148. Otherwise, the switch control signal causes the switch toconnect the average DC level of the input signal to the switch output.Thus, the digital filter 58 effectively blanks non-periodic noise fromthe input signal while passing the desired periodic noise impulses frompower line sparks.

The processor 126 is programmed to implement a method for blankingnon-periodic noise (the process) according to the preferred embodimentof the present invention. The steps of the process are generallyillustrated in flow chart form by FIG. 4.

Basically, the process includes two phases: playback and record. In theplayback phase, a historical determination is made to see whether anoise impulse was detected during a corresponding sample period of thepreceding cycle. If so, the historical record is further checked todetermine whether there was a second historical pulse close to the first("close" here meaning within 16 sample periods of the first pulse, asrecorded in the data array). If so, switch 138 is closed to pass thereceived signal to the output 148 for a brief interval. In the second,record, phase of the process, the current input signal is sampled and adata bit is written to the data array: a "1" if a noise impulse wasdetected, a "0" otherwise. This record serves as the historical recordduring the next cycle (at which time it is overwritten). The foregoingwill become clearer from the following discussion of FIG. 4.

In a first initialization step 160 of the process, the processor 126clears a data array. As described above, the data array will be used tostore a history of the times that noise impulses occurred during a 60 Hzcycle. Each bit of the array stores an indication of whether a noiseimpulse was detected during one of the 120 sample periods during theprevious 60 Hz cycle. In the preferred embodiment, however, the dataarray actually comprises an array of only 116 bits. The array isshortened from 120 to 116 bits to anticipate the interval during which anoise impulse is expected and to account for delays in the sampling ofnoise impulses in the input signal.

Next in the flow chart is an idling step 162 in which the processoridles until the completion of a 139 microsecond sample period. When asample period is complete, the processor proceeds to the step 164 ofdetermining whether a first noise impulse occurred one 60 Hz cycle (16.7milliseconds) previously. (As detailed below, this is somewhat of anoversimplification; the step more accurately entails examining a periodof time centered about a time approximately 16.7 millisecondspreviously.) The data array is consulted to make this determination.Each bit of the data array stores an indication of whether a noiseimpulse was detected during a sample period of the previous cycle. Thebit corresponding to the sample period which occurred 16.7 millisecondspreviously (the critical sample period) will be set if a noise impulseoccurred in that sample period.

If a noise impulse is determined to have occurred in the critical sampleperiod, the processor then determines whether a second noise impulseoccurred within 16 sample periods before the first noise impulse in thestep 166. Since a noise impulse may overlap two consecutive sampleperiods, the processor actually does not check for the occurrence of anoise impulse in the adjacent sample period, but does check for theoccurrence of a noise impulse during the second through sixteenthpreceding sample periods. Since, in the preferred embodiment, the dataarray maintains a history of only the previous 116 sample periods(approximately one 60 Hz cycle), a tail one-shot counter is used todetermine whether a second noise impulse occurred in the second throughsixteenth preceding sample periods. Operation of the tail one-shotcounter is described in more detail below in connection with the flowchart of FIG. 5.

If a second noise impulse did not occur in the 16 sample periodspreceding the first noise impulse, the processor determines whether asecond noise impulse occurred in the 16 sample periods after the firstnoise impulse (step 168). This determination could be made by simplyexamining each of the bits which are 2 to 16 bit locations ahead of thebit corresponding to the first noise impulse. However, to providegreater speed and efficiency, a head one-shot counter is employed in thepreferred embodiment in a manner described below to make thisdetermination.

If a second noise impulse was determined to have occurred within 16sample periods preceding or following the first noise impulse, theprocessor will reach step 168 in which the electronic switch 138 isactuated to pass the input signal for an interval of three sampleperiods. The three sample period duration of the interval in theillustrated embodiment was chosen to allow for small variations in thetiming of noise impulses from a sparking power line (noise sourcejitter) and to allow for timing errors associated with sampling(detecting) the noise impulses. An output counter is loaded to time thisinterval while the processor proceeds to step 172.

In interrupt servicing step 172, the processor services interrupts fromthe noise impulse detection circuitry (comparator 114). In the preferredembodiment, interrupts to the processor are actually masked to preventdisruption of the program. However, as described above, any interruptgenerated since the processor's last interrupt service step is stored inan internal flip-flop connected to the IRQ input 130. In the interruptservicing step 172, the processor checks the flip-flop to determinewhether a noise impulse occurred in the current sample period. If anoise impulse did occur, a bit corresponding to the current sampleperiod is set (step 174). If no noise impulse occurred, the bit iscleared (step 176). Since, in the preferred embodiment, a record of onlythe sample periods in one 60 Hz cycle is kept, the bit associated withthe critical sample period is overwritten in steps 174, 176. After theinterrupt is serviced, the processor returns to the idling step 162.

FIG. 5 is a more detailed flow chart of the preferred embodiment of thenon-periodic noise blanking method (the procedure) of the presentinvention. Block 180 is the initialization step. In this step, theprocessor clears the data array in preparation for the procedure. Block182 is the idling step which acts as a master timing gate. The processoridles at this point in the process until the completion of a sampleperiod (139 microseconds in the preferred embodiment). The processorreturns to this step after executing the entire procedure and before thenext sample period is completed.

After the master timing gate, the first step 184 executed by theprocessor in a sample period is to sample the value stored in theprocessor's internal flip flop at the IRQ input 130. The value in theflip-flop is stored as a temporary variable for use later in theprocedure.

Steps 186-189 are an output servicing routine. In order to continueactuation of the electronic switch 138 for three sample periods whilethe processor continues executing the procedure, the processor musttrack the duration of actuation. Thus, when the processor initiallydetermines that the switch is to be actuated, the processor simply setsan output counter to three. In step 186, the processor checks the outputcounter. If the counter is zero, the processor turns off the switchcontrol signal. As described above, this serves to blank the inputsignal to its average DC level. If, however, the output counter is notzero, the processor turns on the switch control signal (step 188) anddecrements the output counter (step 189). In the preferred embodiment,the output counter is implemented using a byte of the processor'sinternal memory. The initial value of three is simply loaded into thememory initially and the value is later decremented as described above.The output counter could also be implemented using a dedicated hardwarecounter.

In steps 192-195, the processor executes a routine for updating the headone-shot counter. As described above, the head one-shot counter aids indetermining whether a second noise impulse was detected in the sixteensample periods after the critical sample period (the sample period whichoccurred one 60 Hz cycle previous to the current sample period). Forthis and other purposes during the procedure, the processor maintainstwo pointers to bit locations in the data array. A current pointer isassociated with the bit location in which an indication of whether anoise impulse occurred in the current sample interval will be stored.Until that indication is stored, that bit location will still havestored in it an indication of whether a noise impulse occurred duringthe critical sample period. The processor also maintains a head pointerwhich is associated with a bit location that is sixteen bit locationsahead of the bit location associated with the current pointer.

In step 192, the processor checks the bit location associated with thehead pointer. If the bit is set indicating that a noise impulse occurredduring the corresponding sample period, the processor loads the headone-shot counter with the number sixteen. If, however, the bit is clear,the processor proceeds to step 194 in which the value in the headone-shot counter is checked. If the value in the head one-shot counteris non-zero, the counter is decremented in step 195. After execution ofthe routine, the head one-shot counter will indicate by having anon-zero value that a noise impulse occurred between 2 to 16 sampleperiods after the critical sample period. A zero in the head one-shotcounter indicates that no noise impulses occurred in that interval.

The head pointer is updated in steps 198-199. In step 198, the processorcompares the head pointer to its limit which is the pointer value of thelast bit location in the data array. If the head pointer is at thelimit, the processor resets the head pointer to the pointer value of thefirst bit location in the data array (step 199). Otherwise, the headpointer is incremented to point to the next adjacent bit location (step200).

In the next step 204, the processor checks the bit location associatedwith the current pointer. If the bit is clear indicating that no noiseimpulse was detected in the critical sample period, the processordecrements the tail one-shot counter. The tail one-shot counterindicates whether a second noise impulse occurred within 2-16 sampleperiods preceding the sample period one 60 Hz cycle previous. First, instep 206, the processor checks to see if the tail one-shot counter isnon-zero. If the tail one-shot counter is non-zero, the processordecrements it in step 207 and proceeds to step 208 without setting theoutput counter.

However, if the current bit is set indicating that a first noise impulsewas detected in the critical sample period, the processor checks whetherthe tail one-shot counter is non-zero (step 210). If the tail one-shotcounter is non-zero, indicating a noise impulse was also detected in thesixteen sample periods preceding the critical sample period, theprocessor checks whether the tail one-shot counter has a value ofsixteen (step 212). A value of sixteen in the tail one-shot counterindicates that a noise impulse was detected in the sample periodimmediately preceding the critical sample period and may be anoverlapping of the same first noise impulse in both the sample periods.Accordingly, if the tail one-shot counter has a value of sixteen, thetail one-shot counter is left with a value of sixteen (step 214) and thehead one-shot counter is checked for a non-zero value (step 216).

If the head one-shot counter is zero (step 216), then there was nosecond noise impulse detected within sixteen sample periods of the firstnoise impulse. Since no noise impulses were detected within 16 sampleperiods before or after the first noise impulse, the processor proceedsto step 208 without setting the output counter. If the head one-shotcounter is non-zero, however, a second noise impulse was detected in thesixteen sample periods following the first noise impulse. Therefore, theprocessor sets the output counter to three in step 218 so that a switchcontrol signal will be turned on in step 188 of the following sampleperiod.

If in step 212, the tail one-shot counter is determined to be non-zeroand not equal to sixteen, the processor executes steps 220 and 222. Anon-zero and not sixteen value in the tail one-shot counter indicatesthat a second noise impulse was detected in the sixteen sample periodspreceding the critical sample period. The processor, therefore, sets theoutput counter to three in step 222 and loads the tail one-shot counterwith sixteen in step 220 for subsequent sample periods.

Next, the processor executes a routine for recording an indication ofwhether a noise impulse occurred during the current sample period in thebit location associated with the current pointer. In step 208, thetemporary variable in which the contents of the processor's internalflip-flop were stored is checked. If the temporary variable is set, thenthe bit associated with the current pointer is also set (step 226). Ifthe temporary variable is clear, then the bit associated with thecurrent pointer is also cleared (step 228).

Finally, the processor executes a routine for updating the currentpointer. In step 230, the processor checks whether the current pointeris at its upper limit. If the current pointer is at its limit, theprocessor resets the current pointer to its lower limit in step 232.Otherwise, the processor increments the current pointer in step 234.After updating the current pointer the processor returns to the mastertiming gate 182 to repeat the procedure for the next sample period.

Having illustrated and described the principles of my invention withreference to a preferred embodiment, it should be apparent to thoseskilled in the art that the invention can be modified in arrangement anddetail without departing from such principles. For example, while theinvention has been illustrated with reference to a system designed topass 60 Hz power line noise and block all other signals, it will berecognized that the same principles can be applied to block 60 Hz powerline noise while passing all other signals. Such latter systems findwidespread applicability in eliminating interference in television andother communication receivers.

Similarly, while the illustrated invention has been described asblocking undesired signals by opening a switch in the circuit path, itwill be recognized that a similar effect can be achieved by closing aswitch between the circuit path and ground, shunting the undesiredsignal to ground.

Still further, while the invention has been illustrated with referenceto a system in which the signal switching is done in a relatively narrowbandwidth portion of the circuitry, it will be recognized that thisfunction can advantageously be performed in other parts of the circuit.For example, if the switching is done in a wide bandwidth portion of thecircuit, much faster switching intervals can be realized, minimizingdisruption of a signal being filtered. This latter arrangement findsparticular utility in noise blankers for television receivers, wherein anoise pulse might be blanked within a period much shorter than a singleline on the television screen. In such applications, it is sometimesdesirable to have a length of coaxial cable to serve as a delay line, sothat the noise detection can be effected in an undelayed signal path,and then act to blank detected noise from the delayed signal line.

While the invention has been illustrated with reference to a systemtailored for 60 Hz noise, it will be recognized that similar systems canbe implemented for any other frequency. Indeed, with the advances indigital processing techniques and hardware, it is possible to implementa system that detects the frequency of a periodic noise signal andadapts operation of filter/blanker circuitry according to the presentinvention to act on noise of arbitrary frequency.

While the invention has been illustrated as including a comparator todetect noise impulses, it will be recognized that noise impulses can bedetected in other ways. For example, noise impulses typically have avery fast rise time--as fast as the bandwidth of the circuitry permits.This characteristic of a noise impulse, rather than its amplitude, canbe used in detection.

As a further example of alternatives to the comparator based noisedetector, a multi-digit analog to digital converter and signal processorcould instead be used to determine an average noise level of the inputsignal and detect noise impulses which deviate significantly therefrom.

As a still further example of alternatives to the illustrated comparatorbased noise detector, the threshold level of the comparator could bedetermined by AC coupling rather than the illustrated DC coupledmultiplier threshold determination. With AC coupling, a running averageof the peak-to-peak voltage of the input signal is determined. Thisaverage peak-to-peak voltage is multiplied by a constant to calculate anadaptive threshold value.

The method of distinguishing power line noise from other periodic noiseis also susceptible to a number of variations. While the illustratedembodiment examines two successive cycles for indicia characteristic ofpower line sparking, other embodiments may examine three or more cycles,not necessarily successive. Further, other indicia may be used todistinguish power line sparking from other noise. For example,

In view of the many possible embodiments to which the principles of myinvention may be put, it should be recognized that the detailedembodiments are illustrative only and should not be taken as limitingthe scope of my invention. Rather, I claim as my invention all suchembodiments as may come within the scope and spirit of the followingclaims and equivalents thereto.

I claim:
 1. In an apparatus for locating a source of power line arcing,the apparatus including;an antenna input port; an RF receiver having aninput coupled to the antenna input port and providing an output signalat an output thereof; a noise detector having an input coupled to theoutput of the RF receiver; an improvement wherein the noise detectorincludes discriminating means coupled to the RF receiver output fordiscriminating impulse noise due to power line arcing from other impulsenoise having a periodicity matching that of power line arcing.
 2. Theapparatus of claim 1 in which the discriminating means includes meansfor detecting groups of noise pulses, wherein said groups recur with aperiodicity matching that of power line arcing.
 3. The apparatus ofclaim 2 in which said groups of noise pulses each comprises at least twopulses occurring within an interval of less than two milliseconds. 4.The apparatus of claim 1 in which the noise detector further includesadaptive threshold filtering means for comparing an instantaneous valueof the RF receiver output signal with an adaptive threshold value,wherein the adaptive threshold value is an average value of the RFreceiver output signal scaled by a scaling factor, said scaling factorbeing at least three.
 5. A method of distinguishing noise havingcharacteristics of power line arcing from other noise having the sameperiodicity as power line arcing, the method comprising:providing aninput signal to be processed; detecting the presence of two or morenoise impulses within a predetermined brief interval in said inputsignal, said brief interval being an interval during which a power linewaveform is near an extremum; a fixed period following detection of saidtwo or more noise impulses, providing a control signal related theretoto an electronic switch circuit, said electronic switch circuit servingto control passage of a signal related to said input signal to anoutput.
 6. The method of claim 5 in which the brief interval isapproximately 2 milliseconds.
 7. In a method of locating an arcing faultin a power distribution system, the power distribution system conveyingan AC signal that reaches voltage extrema with a fixed periodicity, themethod including receiving a wideband RF signal, analyzing the RF signalto identify noise pulses therein, and processing same to identify noisepulses that recur at integral multiples of said periodicity, animprovement allowing discrimination of noise pulses due to powerdistribution system arcing from other noise pulses having a likeperiodicity, comprising:detecting noise pulses that occur in groups oftwo or more during a brief interval coinciding with said extrema; andidentifying said groups of detected noise pulses as noise pulses due topower distribution arcing.
 8. The method of claim 7 whichincludes:producing a datum upon the detected occurrence of said group ofidentified noise pulses; and a fixed period after the detectedoccurrence of the identified noise pulses, using said datum incontrolling a gating circuit to pass any noise pulses then present inthe received signal to a noise discriminator output.
 9. The method ofclaim 7 in which the analyzing step includes comparing an instantaneousvalue of the received signal with an adaptive threshold value, whereinthe adaptive threshold value is an average value of the received signalscaled by a scaling factor, said scaling factor being at least three.